In recent years, a substantial amount of effort has been devoted to improving short circuiting arc welding by controlling the welding current and/or arc voltage during different portions of a welding cycle constituting a short circuit condition followed by an arcing condition. During the short circuit condition, a molten metal ball formed on the end of the advancing welding wire engages the molten metal pool on the workpiece causing a high current flow through the consumable welding wire and molten metal ball. This short circuit condition is terminated by an electrical pinch action causing the metal forming the molten ball on the wire to electrically constrict and then break away from the welding wire in an explosion type action often referred to as a "fuse" or "the fuse". Controlling current flow during the short circuit portion of the welding cycle is accomplished by the power supply control circuit. In addition, a premonition circuit is usually provided so that a given increase in dv/dt signals the imminent formation of the fuse. Consequently, the welding current can be dropped to a background level I.sub.B or lower immediately before the fuse occurs. In this fashion, the energy of the fuse during each welding cycle is drastically reduced. This reduces spatter at the termination of the short circuit condition. Various circuits for controlling the current flow during the short circuit portion or condition of the welding cycle are known in the art as spatter control circuits since the fuse is considered to be the primary source of spatter during short circuiting arc welding. In applicant's two copending applications, incorporated by reference herein, other spatter producing dynamics of the welding process were recognized and prevented or modified by novel control concepts. One aspect developed by applicants was to provide a high energy pulse following a slight time delay after the fuse so that the arcing condition subsequent to the fuse could be initiated by a high energy current pulse sometimes referred to as a "plasma boost" pulse. By using a high energy plasma boost current pulse immediately upon initiation of an arcing condition in the welding cycle, melting by anode heating at the tip of the welding wire being fed toward the molten metal pool on the workpiece occurred rapidly. This rapid melting allowed formation of a molten metal ball on the end of the wire of uniform size which was then moved toward the pool of molten metal as the wire was fed toward the workpiece. After the plasma boost pulse of current, a background current I.sub.B was passed through the arc to maintain the molten condition of the molten ball. By controlling the current and using a fixed time for the plasma boost pulse, the energy in the plasma boost pulse was regulated. The end of the wire was melted to form a molten metal ball having a somewhat uniform size based upon an amount of energy applied during the power boost current pulse. Thereafter, the arc was operated at a background current level maintaining a molten condition until the short circuit occurred.
Utilizing these prior concepts, which have indeed substantially reduced spatter, a constant voltage control circuit during the plasma boost pulse caused a high current flow during the pulse. This tended to drive the pool away from the inwardly moving molten metal ball. Should the pool be shifted by the energy of the arc, a slight contact could occur at a location spaced from the center of the arc. This short during the plasma boost pulse caused a relatively large spatter event. Thus, using a constant voltage for the power boost current pulse allowed a high current to drive the pool away from the ball which, by fluid dynamics, sometimes tended to cause a wave effect resulting in momentary shorts. To overcome this difficulty, a variable voltage power control circuit has been suggested to maintain a constant current during the plasma boost current pulse. This concept increased the frequency of random shorting during the arcing condition, but each short had a lesser amount of energy to be released. The variable voltage concept employing a constant current condition allowed momentary shorts of less energy. In summary, using constant current or constant voltage during the power boost cycle either increased the frequency of momentary shorts during the arcing condition or their ferocity.
By using a plasma boost pulse having a fixed time, a different amount of energy was introduced into the molten metal ball as the stick-out of the consumable electrode or welding wire varied. Thus, prior systems employing fixed time in the plasma boost current pulse could be used for automatic welding; however, semi-automatic welding wherein manual manipulation changed the extension presented difficulty. The plasma boost current pulse sometimes did not create enough heating on the end of the wire for melting. This caused stubbing. In addition, the duration of the welding cycle was not constant over long periods of time since there was substantial variations in the initiation of the short circuit condition of the individual cycles.